In recent years, several cellular networks for wireless communication have been established to provide radio coverage for serving wireless terminals in different areas. The cellular networks are constantly developed to provide increasingly better coverage and capacity to meet the demands from subscribers using services and advanced terminals, e.g. smartphones and tablets, which require more and more bandwidth and resources for data transport. As a result, it is common to configure a network with different cell sizes to provide needed capacity and flexibility depending on expected traffic intensity in different areas, commonly referred to as a heterogeneous network. Such a network typically comprises hierarchical cell structures including large macro cells covering relatively large areas of kilometer size, and also smaller cells of a few meters size, e.g. micro cells, pico cells and femto cells, to mention some customary examples. The smaller cells are added to the macro cells in an overlapping fashion to increase the capacity locally in so-called “hot spot” areas.
In this disclosure, the term base station represents a radio node of a cellular or mobile network which node is capable of communicating with wireless terminals over radio channels. Depending on the terminology used, a base station may also be called NodeB, eNodeB, eNB, base transceiver station, and so forth. Further, the term “terminal” represents any wireless terminal or device capable of radio communication with a base station in a cellular network.
The different types of cells mentioned above thus have base stations serving mobile terminals to enable radio transmissions in the cells, including downlink signals from the base stations to the terminals and uplink signals from the terminals to the base stations. It can easily be understood that in a large cell, high transmission power is required due to the relatively long distances between the communicating nodes, i.e. between terminals and their serving base stations, while considerably lower transmission power is sufficient in a small cell since the communicating nodes are closer to one another.
Interference is a well-known problem that occurs when nearby radio transmissions disturb the reception of downlink signals in a terminal or uplink signals in a base station. The interfering nearby radio transmissions may contain data and/or control information. So-called inter-cell interference occurs when transmissions in one cell disturb signal reception in another adjacent cell. Numerous solutions exist for avoiding such interference e.g. by coordinating transmissions in adjacent cells and/or by configuring the radio coverage of the cells within a network. It is also possible to change base station antenna azimuth, apply filters, etc., to reduce or avoid interfering signals at the receiver.
However, more than one cellular network may cover basically the same area or adjacent areas, e.g. different networks controlled by different operators, and it may be a problem that radio transmissions in one network can cause interference to signal reception in another network, which can be referred to as “inter-system interference” or “inter-network interference”. Some examples of how such inter-system interference can occur are illustrated in FIGS. 1a-1c, where reception of signals in a first cellular network “1” is interfered by transmission in another cellular network “2”. In this respect, networks 1 and 2 can be seen as a “victim” network and an “aggressor” network, respectively, even though interference may at the same time arise in the opposite direction from network 1 to network 2 as well. In this simplified illustration, the networks 1 and 2 are shown geographically apart from one another although in reality they may have more or less overlapping coverage areas. Further, the networks 1 and 2 may have a mix of large and small cells, schematically suggested by small and large base stations in the figure, where the cells may be arranged in a hierarchical fashion and there is typically no coordination of cell configurations between different networks.
In a first example scenario shown in FIG. 1a, a terminal T2 in the aggressor network 2 sends uplink signals UL2 to a serving base station BS2, while at the same time a terminal T1 in the victim network 1 receives downlink signals DL1 from a serving base station BS1. In this case, the terminals T1 and T2 happen to be close to one another while terminal T2 is relatively far away from its serving base station BS2, e.g. in a macro cell, and T2 must therefore transmit its uplink signals UL2 with quite high power. This transmission therefore causes interference “I”, indicated by a dashed arrow, to the reception of downlink signals DL1 in the nearby terminal T1.
In a second example scenario shown in FIG. 1b, a terminal T2 in the aggressor network 2 likewise sends uplink signals UL2 to a base station BS2, while at the same time a base station BS1 in the victim network 1 receives uplink signals UL1 from a terminal T1. In this case, terminal T2 happens to be close to base station BS1 but far away from its serving base station BS2, e.g. in a macro cell, and T2 must therefore transmit its uplink signals UL2 with quite high power. This transmission therefore causes interference “I” to the reception of uplink signals UL1 in base station BS1. This interference may be particularly severe in case the base station BS1 has its antenna, e.g. mounted on top of a tower or on a wall, close to the ground where terminal T2 is situated.
In a third example scenario shown in FIG. 1c, a base station BS2 in the aggressor network 2 sends downlink signals DL2 to a terminal T2, while at the same time a terminal T1 in the victim network 1 receives downlink signals DL1 from a base station BS1a. In this case, terminal T1 happens to be close to base station BS2 which is far away from the served terminal T2, e.g. in a macro cell, and BS2 must therefore transmit its downlink signals DL2 with quite high power. This transmission therefore causes interference “Ia” to the reception of downlink signals DL1 in terminal T1 and may also cause interference “Ib” in another base station BS1b in the victim network 1, if BS1b is located close enough to BS2. As in FIG. 2, the interference Ia may be particularly severe if BS1 has a low tower with its antenna close to the ground where T1 is situated. Further, if base station BS1b in FIG. 1c, or base station BS1 in FIG. 1b, serves a small cell, e.g. a micro, pico or femto cell, it may have a relatively small and rudimentary receiver with reduced hardware requirements making it sensitive to interference, thus heightening the problem of inter-system interference.
Normally, two networks controlled by different operators use different parts or frequency ranges of the radio spectrum, referred to as allocated bandwidth, such that no transmission should occur on the same frequency in two networks. Nevertheless, radio transmission within a certain frequency range typically means that energy is also emitted to some extent on contiguous frequencies outside, i.e. below and above, the used frequency range, which may cause so-called “out-of-band” interference from one network to another, which is illustrated by a power-frequency diagram in FIG. 2. This shows that one output power distribution over frequency used in an aggressor network, shown by a full line 200, can cause potentially harmful interference in a victim network using another output power distribution over frequency, shown by a dashed line 202.
Even though the two networks transmit most of their power in separated frequency ranges, i.e. within their respective allocated bandwidths which are separated from one another, some power from the aggressor network still “leaks out” out-of-band coinciding with the frequency range allocated to the victim network. As a result, the “noise floor” experienced in the victim network, shown by a dotted line 204, is raised from a background level 206 of thermal noise and own interference to the level of a power 208 leaked from the aggressor network, such that the Signal-to-Interference and Noise Ratio (SINR) is reduced which in turn can deteriorate quality, coverage and capacity in the victim network. As mentioned above, both of two neighboring networks can suffer from such inter-system interference from one another at the same time, so the above explanation and examples may be valid in both directions. It is thus a problem that the cause for such a reduced performance due to inter-system interference is difficult to detect and that it may be a waste of resources and efforts to take actions for reducing inter-system interference when not really needed.